The present invention relates to an abnormality diagnosis method and apparatus for a separable transformer, and more particularly, to an abnormality diagnosis method and apparatus for determining occurrence of short-circuit or wire breakage in a closed circuit including a coil of a separable transformer and a load connected thereto.
A separable transformer, comprising primary and secondary cores arranged for relative rotation and primary and secondary coils wound thereon so as to be electromagnetically coupled to each other, is suitable to transmit power or an electric signal between relatively rotatable members.
FIG. 7 shows a separable transformer 1 having a primary coil 11 and a secondary coil 12 connected to an electronic circuit 2 and a load 3, respectively. The separable transformer 1 serves to receive power or an electric signal from the electronic circuit 2 and transmit the same to the load 3. For example, the separable transformer 1 is used as a rotary connector for establishing, in non-contact manner, electrical connection between a stator fixed to a stationary member of a steering apparatus of an automobile and a rotor fixed to a steering wheel to be rotatable relative to the stator. In this case, the primary and secondary coils 11, 12 are fixed individually to the stator and the rotor, a sensor is connected to the input side of the electronic circuit 2, and a load 3 is constituted by, for example, a heating wire for an airbag received in the steering wheel. The electronic circuit 2 is adapted to supply the primary coil 11 of the separable transformer 1 with detonating power for inflating the airbag when it determines based on a signal from the sensor that the automobile has received large impact. The power is transmitted from the primary transformer 11 through the secondary transformer 12 to the heating wire 3, so that the heating wire 3 is heated to ignite a detonating agent, thereby inflating the airbag.
As understood from the above-described example, the separable transformer requires high operation reliability. On the other hand, a wire or an electric connector is used therein to establish the connection between the primary coil of the separable transformer and the electronic circuit and between the secondary coil and the load. Thus abnormalities can occur, such as connection failure at the electric connector and breakage of the wire.
For a rotary connector having a rotor and a stator electrically interconnected through a flat cable spirally arranged there_between a diagnosis method is known, in which a connection failure at an electric connector and a wire breakage failure at the flat cable are determined based on a resistance value between electric connector terminals or between wires of the flat cable. By this method, an abnormality can be detected at an early stage so as to take necessary steps.
Unlike the flat-cable type rotary connector, the separable transformer, having electromagnetically intercoupled primary and secondary coils between which an ac component is transmitted but a dc component is not transmitted, makes it difficult to determine an occurrence of abnormality based on the resistance value. Specifically, with use of a diagnosis unit connected to the primary coil of the separable transformer for measuring the resistance value across the primary coil, it is impossible to detect an abnormality such as a wire breakage failure or a connection failure at an electric connector used to connect the separable transformer and the load. In the case of the separable transformer, therefore, it is difficult to determine an abnormality in electric connection of the separable transformer and the load by the conventional method.
The object of the present invention is to provide an abnormality diagnosis method and apparatus for diagnosing an abnormality in electric connection of a separable transformer and a load.
In order to attain the object, according to one aspect of the present invention, there is provided an abnormality diagnosis method for a separable transformer having primary and secondary coils thereof adapted to be electromagnetically coupled to each other and wound individually on primary and secondary cores that are arranged to face each other with a predetermined gap there_between for relative rotation. The diagnosis method comprises the steps of (a) measuring mutual induction signals each generated in the primary coil of the separable transformer when an ac signal having a corresponding one of different frequencies is applied to the primary coil, (b) determining a frequency characteristic of mutual induction signal based on the mutual induction signals measured individually at the different frequencies of the ac signal in the step (a), and (c) determining an occurrence of a short-circuit or wire breakage failure in a closed circuit including the secondary coil of the separable transformer and a load connected thereto, based on the frequency characteristic of mutual induction signal determined in the step (b).
The diagnosis method of the present invention determines an abnormality in electric connection on the secondary side of the separable transformer based on mutual induction signals generated on the primary side to which an ac signal or ac power is applied, thereby making it possible to detect, at an early stage, an abnormality in electric connection on the secondary side that cannot be detected by the conventional method that is based on resistance value measurement.
The mutual impedance of the separable transformer as viewed from the primary coil has a different value between when electric connection in the secondary-side closed circuit of the separable transformer is normal and when an abnormality occurs therein, and hence the frequency characteristic of mutual induction signal observed when an abnormality occurs in the closed circuit is different from that for the normal closed circuit. Therefore, it is possible to determine an occurrence of abnormality based on the frequency characteristic of mutual induction signal (for example, a frequency characteristic parameter that characterizes the frequency characteristic), so that an abnormality in the closed circuit can be detected at an early stage and necessary steps can be taken, to improve the operation reliability of the separable transformer.
The separable transformer to which the diagnosis method of the present invention is applied is suitable for use as a rotary connector for electrically connecting a stationary member and a rotary member of a steering apparatus of an automobile, for example, contributing to safety driving of the automobile.
In the diagnosis method of the present invention, desirably, the mutual induction signals are measured while the frequency of the ac signal is varied. According to this desirable method varying the frequency of the ac signal discontinuously or continuously, the mutual induction signals are measured successively in step (a), and the frequency characteristic of mutual induction signal or of a smoothed signal level is determined properly based on the mutual induction signals or smoothed signals corresponding thereto.
Desirably, in step (c), the short-circuit or wire breakage failure in the closed circuit is determined by comparing the frequency characteristic of mutual induction signal determined in step (b) with frequency characteristics of mutual induction signal that are determined in advance individually for closed circuits having a normal electric connection, a wire breakage failure, and a short-circuit failure. With this desirable method, the abnormality determination based on the frequency characteristic of mutual induction signal can be made properly and quickly.
Desirably, the maximum signal level of a smoothed signal and a particular ac signal frequency corresponding thereto are measured by using each of testing separable transformers having the same structure as that of the separable transformer in question and having secondary-side closed circuits one of which is normal and others have wire breakage and short-circuit failures, respectively. The smoothed signal is obtained from a mutual induction signal generated when an ac signal is applied to each testing separable transformer. Based on these measured values, threshold values for predetermined ac signal frequencies, individually corresponding to the particular ac signal frequencies for the normal, wire breakage and short-circuit conditions, are determined in advance. The wire breakage or short-circuit failure in the closed circuit is determined by comparing actual signal levels at the predetermined ac signal frequencies with the threshold values, respectively.
The maximum signal level of smoothed signal and the particular ac signal frequency vary depending on the condition of electric connection in the secondary-side closed circuit of the separable transformer, and hence properly indicate the condition of electric connection in the closed circuit. With this desirable method carrying out the abnormality diagnosis based on the threshold values that are determined, using the testing separable transformers, from the maximum signal levels and the particular ac signal frequencies for the normal, wire breakage and short-circuit conditions, it is possible to properly determine an abnormality in electric connection in the secondary-side closed circuit of the separable transformer.
Desirably, an abnormality is determined based on an amount of change in signal level of the smoothed signal in a frequency region including the particular frequencies for the normal, wire breakage and short-circuit conditions.
The amount of change in the smoothed signal level properly indicates the connection condition in the secondary-side closed circuit of the separable transformer. With this desirable aspect making the abnormality determination based on the amount of change in the smoothed signal level, abnormalities in electric connection in the closed circuit can be determined properly.
According to another aspect of the present invention, there is provided an abnormality diagnosis apparatus for a separable transformer having primary and secondary coils thereof adapted to be electromagnetically coupled to each other and wound individually on primary and secondary cores that are arranged to face each other with a predetermined gap there_between for relative rotation.
The abnormality diagnosis apparatus comprises signal applying means for successively applying ac signals having different frequencies from one another to the primary coil of the separable transformer; signal detecting means for detecting mutual induction signals each generated in the primary coil when a corresponding one of the ac signals is applied to the primary coil, thereby detecting the mutual induction signals; and abnormality determining means for successively detecting frequencies of the ac signals and for determining an occurrence of a short-circuit or wire breakage failure in a closed circuit including the secondary coil of the separable transformer and a load connected thereto, based on a frequency characteristic of mutual induction signal determined from the frequencies of the ac signals and the mutual induction signals detected by said signal detecting means.
With this diagnosis apparatus adapted to determine an abnormality in electric connection on the secondary side of the separable transformer based on the mutual induction signals generated on the primary side when ac signal are applied to the primary side of the separable transformer, it is possible to detect, at an early stage, an abnormality in electric connection on the secondary side that cannot be detected by the conventional method which is based on resistance value measurement, whereby necessary steps can be taken. Thus, the separable transformer provided with the diagnosis apparatus of the present invention has an improved operation reliability.
In preferred diagnosis apparatuses of the present invention, the signal detecting means and the abnormality determining means are configured to achieve functions similar to those achieved by preferred diagnosis methods of the present invention, thereby properly diagnosing the presence or absence of an abnormality in the secondary-side closed circuit of the separable transformer.